Just be sure it's really going down to 12-13 watts. If your device doesn't have voltage step down, it's likely actually firing the 0.5 ohm coil somewhere between 21-25 watts. If you're talking about the 1.2 ohm coil, never mind.
Actually, if you're talking about the 1.2 ohm coil and using a device that doesn't have true voltage step down, the lowest it will fire is somewhere between 12-15 watts.
As a side note, I get the best life out of the original ST and Atlantis heads by ramping them up in wattage gradually (and of course, soaking the hell out of them before firing them the first time). I prefer to run them at the high end of their wattage range, but I'll spend several minutes working up to that.